Massive-MIMO is a technique for performing multi-user MIMO transmission using a large number of antenna elements, and a plurality of configurations are devised such as a configuration in which the directivity of an antenna is digitally created by controlling these large number of antenna elements, or a configuration in which the direction of a beam is controlled using an analog phase shifter.
As a measurement of an antenna having a strong directivity such as an antenna which is used in this Massive-MIMO, a near field measurement (NFM) for calculating a far field directivity from the near electromagnetic field of an antenna on the basis of an electromagnetic field theory has been known.
In the near field measurement, since an electromagnetic field is measured in the vicinity of an antenna, there is an advantage in that a loss of electromagnetic waves due to a space is small, and that the diagnosis of an antenna can be performed from not only a directivity but also the near field distribution of the antenna.
Generally, as shown in FIG. 8, among regions of an electromagnetic field which is radiated from the aperture plane of an antenna, a region adjacent to an antenna aperture is a reactive near field region (extreme near field) mainly containing electromagnetic field components that do not contribute to radiation, and a region having no change in directivity depending on a distance from the antenna aperture is called a radiation far field region (far field). The directivity of an antenna to be generally represented refers to directivity measured in this radiation far field region.
The far field is specified as a position away by more than a distance R which satisfies the following expression with respect to a maximum diameter D (aperture dimension) of an antenna.R>2D2/λ  (1)
Here, λ is a free space wavelength. In addition, when a gain of a transmitting antenna is set to Gt, a gain of the receiving antenna is set to Gr, and transmission power is set to be Wt, maximum power Wa capable of being received by a receiving antenna in a free space is as follows.Wa=(λ/4πR)2·Gt·Gr·Wt  (2)
Therefore, in an antenna having a large aperture plane with a high gain, the distance R increases, and attenuation in a space increases. Further, in a millimeter-wave band, since a free space wavelength λdecreases in size, there is a problem in that the amount of attenuation further increases, and that it is not likely to measure a low-level side lobe.
A radiation near field region (near field) which is a region located between the reactive near field region and the radiation far field region is a region having a change in directivity in accordance with a distance. In the NFM described above, an electromagnetic field is measured in this radiation near field region, and directivity in a far field is obtained by calculation.
Specifically, the vicinity of an antenna to which a predetermined signal is supplied is scanned by a probe antenna, and the distribution of amplitudes and phases for each scanning position is obtained from a signal received by the probe antenna, thereby allowing directivity at infinity to be obtained by data processing from this distribution. Since a measurement in the vicinity of an antenna is performed, the amount of attenuation in a space is small, and thus it is possible to perform a high-accuracy measurement as compared to a far field measurement.
The NFM is divided into a plurality of types depending on a range in which the vicinity of a test antenna is scanned, but is advantageous to an antenna having a high gain, and a plane NFM which is easy of data processing is widely used.
FIG. 9 shows a configuration of a measurement device 10 that obtains the directivity of a test antenna 1 using the plane NFM. This measurement device 10 includes a support 11 that supports the test antenna 1 in a state where its radiation plane is directed toward a predetermined direction, a probe antenna 12 for receiving electromagnetic waves which are output from the test antenna 1, and a probe scanning mechanism 13 that moves the probe antenna 12 in X and Y directions within a neighboring measurement plane facing the radiation plane of the test antenna 1.
In addition, the measurement device 10 includes a measurement control unit 24 that controls a signal generator 21 for assigning a measuring signal to the test antenna 1, an amplitude and phase detector 22 for detecting information of an amplitude and a phase from a received signal of the probe antenna 12, and a probe scanning mechanism 13, receives an output of the amplitude and phase detector 22 while scanning the position of the probe antenna 12 at a predetermined pitch within a measurement plane P, and obtains a far field directivity of the test antenna 1 from the distribution of amplitudes and phases within the measurement plane P, and a display unit 25 that displays the obtained directivity of the test antenna 1. Meanwhile, as the signal generator 21 and the amplitude and phase detector 22, a network analyzer having their functions can be used, and a personal computer can be used as the measurement control unit 24.
Here, in a case of the NFM, the probe antenna 12 scans the neighboring measurement plane P located away by approximately three wavelengths of a measurement signal from the test antenna 1, and the amplitude and phase of its electric field are detected.
The distribution of amplitudes and phases in the measurement plane P has a form of Fourier transformation of a function which is defined from the directivity of the test antenna 1 and the directivity of the probe antenna 12. In the measurement control unit 24, the function is obtained by inverse Fourier transformation, and then arithmetic processing (probe correction) of removing the directivity of the probe antenna is performed, thereby allowing the directivity of the test antenna 1 to be obtained. In the measurement control unit 24, since data processing can be performed by fast Fourier transformation (FFT), it is possible to calculate the far field directivity of the test antenna 1 at a fast rate.
It is generally known as disclosed in Non-Patent Document 1 that, as described above, the distribution of amplitudes and phase in the measurement plane P has a form of Fourier transformation of a function which is defined from the directivity of the test antenna and the directivity of the probe antenna, its function is obtained by inverse Fourier transformation, and then arithmetic processing (probe correction) of removing the directivity of the probe antenna is performed, thereby allowing the directivity of the test antenna to be obtained.
The NFM in which the directivity of the antenna is obtained in this manner has the following advantages over a far field measurement (FFM).
Since the NFM is a measurement in a short distance, a measurement can be performed even in a case where an anechoic chamber is not used, and a large-scale device is not required. In addition, since a device becomes compact in a millimeter-wave band, a measurement in a simple anechoic chamber installed in a living room can be performed, and it is possible to drastically shorten a time spent in constructing a measurement system which is a problem in a measurement in an anechoic chamber. Further, since a measurement in a region having a small free-space loss is performed, it is possible to obtain measurement results with good accuracy.
Further, in the NFM, the distribution of amplitudes and phases in the vicinity of an antenna is obtained. Therefore, in a case where directivity as designed is not obtained, the cause can be diagnosed. This is a greatly advantage to a phased array antenna such as a massive-MIMO antenna.